Recovery of petroleum



May 22, 1962 J. c. ALLEN RECOVERY 0F PETROLEUM 2 Sheets-Sheet 1 FiledSept. 9, 1957 May 22, 1962 J. c. ALLEN RECOVERY 0R PETROLEUM 2Sheets-Sheet 2 Filed Sept. 9, 1957 United States Patent Office3,[i35,637 Patented May 22, 1962 3,035,637 RECGVERY F PETROLEUM JosephC. Allen, Bellaire, Tex., assignor to Texaco Inc., a corporation of'Delaware Filed Sept. 9, 1957, Ser. No. 682,760 4 Claims. (Cl. 166-9)This invention relates to the recovery of petroleum from undergroundformations.

In the petroleum eld, various so-called secondary recovery processes areemployed to recover the petroleum remaining behind in the petroleumreservoir after the primary producing forces (solution gas drive, waterdrive and the like) have been depleted. Various secondary recoverytechniques have been proposed to effect the recovery of this residualpetroleum. These techniques have included water flooding wherein wateris introduced via an injection Well into the petroleum reservoir todisplace petroleum therefrom toward a production well. Gas flooding orgas pressuring wherein gas is introduced into the reservoir via aninjection well to drive or displace petroleum toward a production wellhas also been employed. Gas sweeping wherein a relatively lean gas ispassed through the reservoir toward a production well has also beensuggested. During this sweeping operation the heavier petroleumcomponents are vaporized into the lean sweeping gas and are produced ingaseous form via the production well. Miscible phase iiooding has alsobeen proposed. In miscible phase liooding a liquefied normally gaseoushydrocarbon, such as LRG., is introduced into the reservoir followed bythe introduction of a driving fluid, such as a gas, eg., natural gas,via an injection well to displace petroleum from the reservoir. Theresulting displaced petroleum is then produced via a production well.

lt has been recognized that capillary forces play an important role'inthe displacement process usually involved in the secondary recovery ofpetroleum from underground formations. lt has been theorized that if theinterfacial tension existing between two separate phases (eg. naturalgas and oil) in a petroleum reservoir could be reduced to zero or almostzero substantially all of the petroleum could be displaced therefrom bya suitable driving uid, viz. natural gas. For example, it has beenspeculated that the addition of suitable surfactant agents to water in awater drive to reduce the interfacial tension between the water and thepetroleum to substantially zero would result in the substantiallycomplete recovery or displacement of the petroleum from the reservoir bythe water drive or ood. Unfortunately, however, from a practical pointof view suitable surfactant agents which so reduce the interfacialtension between the petroleum and water in a water drive process are notavailable. Accordingly, secondary recovery water flood operationsemploying this technique have not been completely successful.

Accordingly, it is an object of this invention to provide an improvedsecondary recovery process for the recovery of petroleum fromunderground petroleum-containing formations.

It is another object of this invention to provide a process wherein asweeping gas, having a critical composition with respect tothe petroleumin the reservoir undergoing treatment, is employed in the secondaryrecovery operation to effect the displacement of petroleum from thereservoir.

Still another object of this invention is to provide an improvedsecondary recovery operation employing a sweeping gas which has acritical composition with respect to petroleum in the reservoir beingswept such that under the formation conditions of temperature andpressure the sweeping gas forms a critical single phase admixture withthe petroleum therein.

yStill another object of this invention is to provide a secondaryrecovery operation wherein the interfacial forces between petroleum andthe sweeping uid (gas) is reduced to zero.

Yet another object of this invention is to provide a secondary recoveryprocess wherein it is possible to recover substantially completely allof the petroleum in the reservoir being swept.

How these and other objects of this invention are accomplished willbecome apparent with reference to the accompanying disclosure anddrawings wherein FIG. l schematically illustrates a secondary recoveryprocess in accordance with the practice of this invention, and whereinFlG. 2 graphically illustrates the practice of this invention as appliedto the secondary recover] of petroleum from an undergroundpetroleum-containing reservoir wherein a gaseous mixture of hydrocarbonshaving a special critical composition with respect to the formationpetroleum is employed as the driving fluid or displacing medium.

Under a given set of conditions of reservoir temperature, pressure andknown composition of reservoir petroleum (oil) and gas there exists acomposition of injected gas such that when combined with the reservoirpetroleum will form a critical mixture of a single phase wherein thereservoir petroleum loses its identity. When this occurs the capillaryforces normally associated with a petroleum producing formation (such asare present at a gas-liquid petroleum interface) are reduced to zero,thereby releasing all the petroleum in the formation or reservoir porespace to the critical mixture. Accordingly, in the practice of thisinvention a gaseous hydrocarbon or mixture of hydrocarbons having acomposition capable of forming a critical single phase admixture withthe petroleum hydrocarbons under the formation conditions of temperatureand pressure is introduced into the formation into contact with thepetroleum hydrocarbons therein to form a critical phase therewith.Following the injec` tion of the gaseous hydrocarbons there isintroduced into the formation a second gaseous hydrocarbon or mixture ofhydrocarbons which has a composition critical with respect to theaforementioned first injected gas so that said critical single phaseadmixture is maintained. By operating in the above-described mannerthere is established a critical state between the reservoirv petroleumhydrocarbons and the rst introduced gaseous mixture of hydrocarbons andthe critical state established between the rst introduced gaseousmixture of hydrocarbons and the formation petroleum or hydrocarbons ismaintained. Accordingly, having the critical state established andmaintained within the petroleum bearing formation in the secondaryrecovery process, the necessary steps for obtaining the optimum indisplacement eiciency have been taken.

Referring now to FIG. 1 of the drawing which schematically illustrates apractice of this invention as applied to the displacement of petroleumfrom an underground reservoir, there is schematically illustratedtherein a section of a petroleum-containing reservoir as it is beingswept or produced in accordance with this invention. More particularly,as indicated in FlG. l a gaseous mixture of hydrocarbons having asuitable composition is introduced at the left hand end of FIG. l andoil produced from the formation is shown recovered at the right hand endof FIG. l. Intermediate the formation between the introduction of thesuitable, ultimate gas and the production of petroleum therefrom thereare schematically illustrated various transition zones which existtherein. Proceeding from right to left zone A illustrates a bank ofpetroleum as displaced from the formation during the treating process.Zone B is a transition zone and comprises petroleum enriched withrespect to intermediate hydrocarbons in the range CM. At the trailingedge of Zone B will be the critical mixture, single phase only,corresponding to the formation conditions of temperature and pressureand brought about by the introduction of a hrst gaseous adr'nixturewhich is critical with respect to the petroleum originally within theformation. Transition zone C is the transition zone between this firstintroduced critical gaseous mixture and a subsequently introduced secondgaseous admixture which is critical with respect to the first introducedgaseous mixture under the formation conditions of temperature andpressure so that a single phase is maintained therein. Zone D has only asingle hydrocarbon phase maintained therein and comprises substantiallyonly the subsequently injected second critical gaseous admixturetogether with some of the first injected driving gas. Transition zones Cand D will have varying sizes depending upon the composition of thepetroleum being displaced and the makeup of the critical displacinggaseous hydrocarbon mixtures. Following zone D is zone E comprisingsubstantially only the final driving gas which usually comprisessubstantially only methane. The composition of this driving gas is suchto maintain only a single hydrocarbon phase in that portion or Zone ofthe formation swept by this gas.

In a special embodiment of the practice of this invention as illustratedin FIG. 1 it is preferred to employ a relatively large amount of thefirst injected gaseous mixture of hydrocarbons which is critical withrespect to the petroleum within the reservoir undergoing treatment, suchthat there exists within the reservoir during the secondary recoveryoperation a substantial portion thereof which is occupied bysubstantially only the first introduced critical gaseous admixture.

Referring now to FIG. 2 of the drawing which graphically illustrates thepractice of this invention, there is illustrated therein by means of athree component composition graph a complex hydrocarbon system such asis found in petroleum reservoirs and the composition and phase changeswhich occur in the practice of this invention.

As indicated in FIG. 2, the triangular graph indicates a relativelycomplex petroleum or oil mixture, such as may be found within apetroleum producing formation, as being made up of three components, (R)methane, (S) hydrocarbons in the range C2 6 and (T) hydrocarbons in therange C7 and higher. Although the designation of a complex petroleumsystem such as occurs in an underground formation as indicated by FIG. 2is somewhat arbitrary, this graphical presentation is very useful inqualitatively and thermodynamically explaining what takes place withinthe petroleum producing formation during the practice of this inventionand approaches actual formation conditions. Conditions illustratedtherein are for a given temperature and pressure, viz. formationtemperature and pressure. In FIG. 2 the formation temperature is 206 F.and the formation pressure is 2,000 p.s.i.a. All compositions noted inFIG. 2 are in mol percent.

That portion of FIG. 2 encompassed by the loop or phase boundary curve Xdelineates the area of compositions wherein two phases exist.Compositions outside of this delineated area exist only in a singlephase. A tie line Y (parallel to base composition line RT) is showncutting across the two phase region encompassed Within line X. The twopoints Y1 and Y2 on line X represent the saturated oil and the saturatedgas, respectively, which would be in equilibrium with each other ifplaced in contact at the temperature and pressure existing for the givendiagram. The limiting tie line is a pointV F representing the critical(only single phase) mixture for the existing temperature and pressure.Mixtures represented by points above and to the left of the saturatedvapor portion of curve X are gas (area V) and mixtures represented bypoints above and to the right of the saturated liquid portion of curve Xare oil (area W). Accordingly, mixtures with greater percentages ofintermediate (C2 6) or those lying above point F are in the zone that iseither a liquid or gas if they can be visibly defined in the presence ofone or the other. These are the areas of critical mixtures. Dashed lineT passing through critical point F of the phase boundary curve X is theline of minimum richness for the injected critical gas and delineatesthe areas of critical mixture lying thereabove.

Considering now the application of the practice of this invention asapplied to a petroleum reservoir which has a composition such as may berepresented by point A within the phase boundary curve X (the reservoirpetroleum may have a composition within or without the two phase regiondelineated by curve X), drawing a line through point A through point Fand extending this line to that side RS of the triangular graph to pointG, which point represents a composition or gaseous mixture ofhydrocarbons containing 65% propane (C2 6) and 35% methane, a gaseousadmixture comprising approximately 2/3 propane and 1/3 methane. It isapparent from FIG. 2 that when the gas of composition G is introduced insucient amount into the formation containing oil having a compositionindicated by point A the composition of the oil within the formation, asit is contacted by and is intermingled with the gas of composition A,will change and will move along line AG in the direction indicated bythe arrowhead thereon to point F which corresponds to the criticalcomposition under the existing formation conditions of temperature andpressure. When point F is reached all phase boundaries between a gas andoil mixture of that composition within the formation disappears and ineffect only one phase exists. From point F a line is extendedhorizontally, parallel to the base line RT representing componentsmethane and C7 and heavier to point H. Point H represents a gaseousadmixture containing approximately 47% propane (C2 6) and 53% methane.

Following the injection of the rst gaseous admixture having acomposition indicated by point G, a second gaseous mixture ofhydrocarbons having the composition indicated by point H (53% methaneand 47% propane) is introduced into the formation. As indicated by FIG.2, the composition of the petroleum at point F within the formationmoves along the line HF in the direction indicated by the arrowheadthereon. Sufficient gaseous mixture having the composition representedby point H is introduced into the formation until the petroleum thereinreaches a composition on line HF indicated by point E. When thepetroleum in the formation reaches the composition indicated by thepoint E there is introduced into the formation a relatively dry gas,comprising substantially only methane, such as a gas having acomposition represented by point K on the base line RT of the threecomponent triangular graph. The composition of the petroleum within theformation at point E would then move along the line EK in the directionof K as indicated by arrowhead on the line EK.

It is thus seen that when an operation is carried out in the mannerdescribed herein the avoidance of the formation of two separate phaseswithin the petroleum formation during the secondary recovery operationis achieved. It is to be noted that line EK does not penetrate or enterinto the two phase boundary region delineated by phase boundary curve Xbut at most passes tangent thereto or outside of phase boundary curve X.

The advantages of carrying out a secondary recovery operation asdescribed hereinabove wherein a portion of the formation petroleum, asit is displaced through the formation, follows the path AFEK would bereadily apparent when one considers the operations which would beinvolved if instead of the selection of a critical gas, such as isrepresented by point G, or a fluid having a composition represented by apoint in the areas V and W of FIG. 2, one were to inject into theformation a substantially pure propane, as would be represented by pointS (C2 6) of the three component phase diagram of FIG. 2. Should propanealone be introduced into the formation the formationuid compositionwould follow the path represented by dashed line AS. It would benecessary to introduce a very substantial amount of propane into theformation, at least sufficient to reach a composition indicated by pointC on line AS. Once the composition represented by point C had beenreached the introduction of a relatively dry gas comprisingsubstantially only methane (eg. composition K on line RT) might becommenced so as to effect a displacement of the formation hydrocarbonsthrough the formation in the critical phase without entering the twophase region delineated by phase boundary curve X, the area encompassedby curve X depending upon formation temperature and pressure, higherformation temperature and pressure tending to compress the two phaseregion P.

The phase boundary curve indicated by line X of FIG. 2 is typical ofreservoir petroleum fiuids. Further, it has been noted that the criticalcomposition of petroleum fluids usually contains about -13 mol percentintermediate hydrocarbons in the range C2 5.

In summary, in accordance with the practice of this invention asecondary recovery operation is carried out in the following manner.There is introduced into the formation a gaseous admixture having acomposition (such as G) such that it forms with the formationhydrocarbons a critical single phase therewith. When a sufcient amountof this first injected gaseous mixture of hydrocarbons has beenintroduced into the formation there is introduced into the formation asecond gaseous mixture of hydrocarbons equivalent to a gaseous mixturepreferably having the composition H or lying along the line HG of FIG.2. A suicient amount of this second injected mix-ture of gaseoushydrocarbons is introduced to form within the formation undergoingtreatment a composition indicated by point E. Thereupon there isintroduced into the formation a relatively lean gas, such as a gascomprising substantially only methane, as indicated by a gas having thecomposition indicated by point K or a point along the line RK or in thearea L of the three component diagram of FIG. 2. By operating in theabove-indicated manner a completely critical single phase displacementof petroleum hydrocarbons can be effected and the benefits derivabletherefrom are obtained.

It is mentioned that those driving gas compositions set forthhereinabove with respect to the operation of the subject invention asillustrated by FIG. 2 are not limitive of this invention but are merelyillustrative thereof. Any suitable gaseous hydrocarbon mixtures such asa gaseous mixture having a composition within the area V of FIG. 2 mightbe employed. It is mentioned, however, that once a critical single phasecomposition has been obtained only a single phase should be maintainedthroughout the formation being swept extending from the injection wellto the foremost edge or boundary of the established critical phase inthe direction of the production well.

The embodiment of this invention described with respect to FIG. 2 is thepreferred embodiment wherein the composition of the first injected gasis selected so that the reservoir petroleum changes composition in astraight line directly to its critical composition as illustrated bypoint F. Likewise, it is preferred that the second injected gas have acomposition such as indicated by point H so that the formed criticalsingle phase, as its composition is changed by contact with a gas ofcomposition H, does not penetrate the two phase region P delineated byphase boundary curve X, but passes at most only tangent thereto asindicated by line EF. Accordingly, a gaseous hydrocarbon mixture havinga composition in the area L would not be suitable in accordance with thepractice of this invention since the initially formed single phase ofcomposition F would revert to a two phase composition in the region Pupon the continued injection of such a gas. Accordingly, the limitingcomposition of the second injected mixture of hydrocarbons is within theareas V and W, and delineated by that portion above dashed line T.

In a particularly preferred embodiment it is preferred to carry out theforegoing operations such that the shortest possible path is taken froma point, such as point A which is representative of the composition ofthe reservoir petroleum, through its critical point F, to point R or Kor an area surrounding the same which corresponds to a gas consistingessentially of substantially only methane without said path penetratingor entering the two phase region P delineated by phase boundary curve X.As is apparent from FIG. 2 only one such path exists for a givencomposition of reservoir petroleum and reservoir conditions oftemperature and pressure but, on the other hand, it is realized thatthere are an infinite number of paths, other than the preferred andshortest path, which might be taken by an operator and still practicethis invention and enjoy and attain the benefits and advantagesderivable therefrom with respect to the efficiency and completeness of asingle phase displacement or fiooding operation.

Although the description of this invention has been made hereinabovewith respect to an injection well and a production well it is realizedthat one or more injection wells may be involved and one or moreproduction wells may be involved.

As will be apparent to those skilled in the art many substitutions,changes and alterations are possible in the practice of this inventionwithout departing from the spirit or scope thereof.

I claim:

l. In a secondary recovery operation of petroleum from an undergroundpetroleum bearing formation wherein a fluid is introduced via aninjection well into said petroleum bearing formation to drive petroleumfrom said formation toward a production well, the improvement whichcomprises employing as said fluid a first gaseous hydrocarbon mixturehaving a composition with respect to said petroleum in said formationsuch that said first gaseous hydrocarbon mixture is capable uponadmixture with said petroleum within said formation of forming therewitha first single phase admixture in the critical state having thecomposition indicated at the critical point of said petroleum withinsaid formation under the formation conditions of temperature andpressure as indicated on a three component methane -C2 6-C7+ phasediagram for said petroleum under formation conditions of temperature andpressure, forming the aforesaid first single phase admixture within saidformation by injecting said rst gaseous hydrocarbon mixture thereinto,subsequently injecting into said formation via said injection well asecond gaseous hydrocarbon mixture having a composition with respect tosaid first single phase admixture such that said second gaseoushydrocarbon mixture is capable upon admixture therewith of maintainingthe resulting admixture in single phase condition under formationconditions of temperature and pressure, said second gaseous hydrocarbonmixture comprising a normally gaseous hydrocarbon and a hydrocarbon inthe molecular weight range of C22-C6 of lesser proportion than containedin said first gaseous hydrocarbon mixture, and thereafter introducinginto said formation a relatively lean gas having a compositioncomprising substantially only methane chosen from outside the 2-phaseregion of said three component phase diagram capable only of mixing withand maintaining the aforesaid resulting admixture in single phasecondition thereby to effect a substantially complete single phasedisplacement of petroleum from said formation.

2. An operation in the process of claim l wherein said first gaseoushydrocarbon mixture comprises substantially methane and a hydrocarbon inthe molecular Weight range CZ-CG.

3. An operation in the process of claim l wherein said ananas? firstgaseous hydrocarbon mixture comprises substantially methane and propane.

4. A petroleum recovery process which comprises introducing into apetroleum bearing formation a rst gaseous hydrocarbon mixture, said rstgaseous hydrocarbon mixture having a composition with respect to saidetroleum within said formation such that under the formation conditionsof temperature and pressure said rst gaseous hydrocarbon mixture forms asingle phase admixture in the critical state with said petroleum Withinsaid formation, the composition of said single phase admixture beingsubstantially the same as the composition indicated at the criticalpoint of said petroleum Within said formation under the formationconditions of temperature and pressure as indicated on a three componentmethane-C2 6-Cf,+ phase diagram for said petroleum under formationconditions of temperature and pressure, forming said single phaseadmixture having the aforesaid composition within said formation byintroducing said first gaseous hydrocarbon mixture thereinto,subsequently introducing into said formation a second gaseoushydrocarbon mixture, said second gaseous hydrocarbon mixture having acomposition with respect to the aforesaid critical state single phaseadmixture formed Within said formation such that said second gaseoushydrocarbon 25 mixture under formation conditions of temperature andpressure is capable only of mixing with the aforesaid critical statesingle phase admixture within said formation and maintaining theresulting mixture in single phase condition, said second gaseoushydrocarbon mixture comprising a normally gaseous hydrocarbon and ahydrocarbon in the molecular Weight range C2-C6 of lesser proportionthan contained in said rst gaseous hydrocarbon mixture, and thereafterintroducing into said formation a relatively lean gas having acomposition comprising substantially only methane chosen from outsidethe Z-phase region of said three component phase diagram capable only ofmixing with and maintaining the aforesaid resulting mixture in singlephase condition thereby to effect a substantially complete single phasedisplacement of petroleum from said formation.

References Cited in the le of this patent UNITED STATES PATENTS2,718,262 Binder Sept. 20, 1955 2,742,089 Morse et al. Apr. 17, 19562,822,872 Rzasa et al. Feb. l1, 1958 2,880,801 Crump Apr. 7, 1959FOREIGN PATENTS 696,524 Great Britain Sept. 2, 1953

1. IN A SECONDARY RECOVERY OPERATION OF PETROLEUM FROM AN UNDERGROUNDPETROLEUM BEARING FORMATION WHEREIN A FLUID IS INTRODUCED VIA ANINJECTION WELL INTO SAID PETROLEUM BEARING FORMATION TO DRIVE PETROLEUMFROM SAID FORMATION TOWARD A PRODUCTION WELL, THE IMPROVEMENT WHICHCOMPRISES EMPLOYING AS SAID FLUID A FIRST GASEOUS HYDROCARBON MIXTUREHAVING A COMPOSITION WITH RESPECT TO SAID PETROLEUM IN SAID FORMATIONSUCH THAT SAID FIRST GASEOUS HYDROCARBON MIXTURE IS CAPABLE UPONADMIXTURE WITH SAID PETROLEUM WITHIN SAID FORMATION OF FORMING THEREWITHA FIRST SINGLE PHASE ADMIXTURE IN THE CRITICAL STATE HAVING THECOMPOSITION INDICATED AT THE CRITICAL POINT OF SAID PETROLEUM WITHINSAID FORMATION UNDER THE FORMATION CONDITIONS OF TEMPERATURE ANDPRESSURE AS INDICATED ON A THREE COMPONENT METHANE -C2-6-C7+ PHASEDIAGRAM FOR SAID PETROLEUM UNDER FORMATION CONDITIONS OF TEMPERATURE ANDPRESSURE, FORMING THE AFORESAID FIRST SINGLE PHASE ADMIXTURE WITHIN SAIDFORMATION BY INJECTING SAID FIRST GASEOUS HYDROCARBON MIXTURE THEREINTO,SUBSEQUENTLY INJECTING INTO SAID FORMATION VIA SAID INJECTION WELL ASECOND GASEOUS HYDROCARBON MIXTURE HAVING A COMPOSITION WITH RESPECT TOSAID FIRST